anything more than idle in gear (ahead
or astern) when maneuvering, means you've screwed up the
approach/undocking.
Is this an absolute? Of course not. It's an ideal you strive for.

Uh no. It depends entirely only the vessel and how it's set up. Some boats
handle fine at idle, some do not. "Striving for" is stupid if the boat
handling won't benefit from it.

Never treat prop walk, wind or current as your enemy....make them your
friend, working for you, not against you

Indeed, since you have no brakes and water doesn't slow you down very much
(aka traction) you need to take advantange of any sources of force present.
Use your prop wash to 'walk' the boat with it. Or keep that spring line
handy to add some extra pivot force in close quarters. Even if practice
eventually teaches you to do without it, you'll have a skill ready to apply
in other situations.